Space Systems Design Studio

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Flux-Pinned Spacecraft Research

Multibody Spacecraft Reconfiguration

With spacecraft modules bound together by magnetic field interactions, components can be disconnected and reconnected with relative ease, allowing multibody spacecraft reconfiguration at the system level. The dependence of flux pinning on magnetic fields does not only mean that spacecraft connected by flux pinning can switch their connections on and off magnetically, but also that spacecraft can shape their magnetic fields to achieve certain motions or behaviors. For example, a symmetric magnetic field creates a flux-pinned interface with a kinematic degree of freedom, the basis for creating a system of non-contacting mechanisms. A multibody spacecraft can specify its joint structure, becoming one of many possible kinematic mechanisms.

In the presence of ambient force fields in the space environment, such as gravitational or magnetic fields, these non-contacting mechanisms will move to a stable equilibrium configuration. From that equilibrium, the spacecraft can select a new set of joint kinematics and move to stable equilibrium again. By controlling the kinematics, rather than the dynamics, of the multibody spacecraft, operators can reconfigure systems in orbit with very little power usage, little control effort, and low risk of inter-body collisions. Our research in this area involves developing algorithms to identify all the possible configurations a multibody spacecraft can reach and testing the performance of this control strategy.

To support this research, we have developed QuIRK: a multibody dynamics toolbox for Matlab. This toolbox interactively models multibody dynamic systems from the Matlab command interface, and is available for download at this web site.

For more information, see:

• Shoer and Peck, "Sequences of Passively Stable Dynamic Equilibria for Hybrid Control of Reconfigurable Spacecraft"
• Wilson, Shoer and Peck, "Demonstration of a Magnetic Locking Flux-Pinned Revolute Joint for Use on CubeSat-Standard Spacecraft"
• Shoer and Peck, "Reconfigurable Spacecraft as Kinematic Mechanisms Based on Flux-Pinning Interactions"

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Multibody Dynamics and Control

For small motions, flux pinning can be modeled as a spring and damper acting in many degrees of freedom. This approximation becomes less accurate as the relative position displacements increase, but it provides the foundation of a convenient linear model of a flux-pinned system's dynamics. Standard linear system analysis techniques can therefore describe the system's properties such as stability, controllability, and observability.

The difference in gravitational attraction between the individual bodies in a multibody space system can shift the relative positions of a static formation away from the desired states if that system does not benefit from appropriate control input or stabilizing dynamics. The additional dynamics due to flux pinning connections between rigid bodies tend to passively stabilize the relative motions of the bodies by stiffening and damping particular modes. Since the stiffness and damping associated with each degree of freedom tend to increase exponentially as the separation distance closes, flux pinning in a multibody space formation sees the most benefit in close proximity formation flight. Typical examples of this type of formation are seen in docking, object manipulation, and in-orbit assembly tasks.

For more information, see:

• Norman, "Stationkeeping and Reconfiguration of a Flux-Pinned Satellite Network"
• Norman and Peck, "Modeling and Properties of a Flux-Pinned Network of Satellites"

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Control of Nonlinear Non-Contacting Interfaces

Flux pinning involves a highly nonlinear potential well that can be exploited to produce desirable passive dynamics for spacecraft equipped with a flux-pinned interface. However, it is useful to be able to control the two flux-pinned spacecraft so that they can achieve relative positions and attitudes beyond the passive equilibrium. The goal of this research is to develop control methods that intelligently exploit the nonlinearities in flux pinning while providing spacecraft operators the flexibility and sensitivity necessary to produce a useful interface.

For more information, see:

• Jones, "Stability and Control of a Flux-Pinned Docking Interface for Spacecraft"

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Formation Control with Other Non-contacting Forces

Flux pinning is only one of several forces that act between the individual craft in a formation without mechanical contact. Another such force is photon pressure from a laser beam, which can act over much larger distances than flux pinning.

Under some conditions, these forces interact between spacecraft only during discrete intervals, such as when two spacecraft in orbits with opposite directions of travel pass one another. Even during such brief encounters, these spacecraft can use non-contacting forces to manipulate their orbits relative to one another. Space Systems Design Studio researchers are developing nonlinear models and hybrid controllers for such maneuvers. Applications include orbit maintenance, orbit raising, and formation station-keeping.

For more information, see:

• Norman, "Stationkeeping and Reconfiguration of a Flux-Pinned Satellite Network"

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